Disc drives read and write information along concentric tracks formed on discs. To locate a particular track on a disc, disc drives typically use embedded servo fields on the disc. These embedded fields are utilized by a servo subsystem to position a head over a particular track. The servo fields are written onto the disc when the disc drive is manufactured and are thereafter read by the disc drive to determine position.
Ideally, a head following the center of a track moves along a perfectly circular path around the disc. However, two types of errors prevent heads from following this ideal path. The first type of error is a written-in error that arises during the creation of the servo fields. Written-in errors occur because the write head used to produce the servo fields does not always follow a perfectly circular path due to, for example, unpredictable pressure effects on the write head from the aerodynamics of its flight over the disc, and from vibrations in the gimbal used to support the head. Because of these written-in errors, a head that perfectly tracks the path followed by the servo write head will not follow a circular path.
The second type of error that prevents circular paths is known as a track following error. Track following errors arise as a head attempts to follow the path defined by the servo fields. The track following errors can be caused by the same aerodynamic and vibrational effects that create written-in errors. In addition, track following errors can arise because the servo system is unable to respond fast enough to high frequency changes in the path defined by the servo fields.
Written-in errors are often referred to as repeatable runout errors (RRO) because they cause the same errors each time the head moves along a track. As track densities increase, these repeatable runout errors begin to limit the track pitch. Specifically, variations between the ideal track path and the actual track path created by the servo fields can result in a track interfering with or squeezing an adjacent track. Track squeeze occurs when the distance of two adjacent tracks written by the servo track writer is smaller than the specified track spacing at certain points. Vibrations during the servo track writing process can cause track squeeze. Track squeeze has to be accounted for as an uncertainty when specifying the track spacing of a disc drive, and therefore, track squeeze limits the maximum achievable track density.
Track misregistration can also be caused by media imperfections. Slight differences of the magnetic properties of the media over the disc surface may cause variations in the magnitude of the servo bursts read by the head. This, in turn, results in a position measurement error and track misregistration.
Referring to diagram 100 in FIG. 1, solid line 102 represents an ideal servo track. Dashed line 104 represents the track center after the servo write process. Because of various disturbances occurring during the servo write process and media imperfections, the track center is not smooth. A disc drive actuator typically would have difficulty following this path.
During the operation of the disc drive, a position measurement signal is generated at each servo burst, and fed into a control system. The control system computes a correction factor or position error signal (PES), which is equivalent to the deviation of the measured actuator position from the desired position. During track following, the position error signal is a direct measure of the track misregistration and includes repeatable and non-repeatable components. The repeatable component, referred to as the repeatable position error signal, includes the repeatable runout written in by the servo track writer (SWRRO), and the disturbance caused by media imperfections. The control system makes use of the position error signal to reposition the head.
If the non-repeatable position error component is neglected, the perfectly circular track center can be followed with zero actuator acceleration. When zero actuator acceleration is achieved (a zero acceleration path or ZAP), track squeeze and track misregistration may be significantly reduced. A basic principle of ZAP correction method is to subtract an appropriate correction factor from the position measurement signal at each servo sample. If the correction factors are determined appropriately, the original zigzag path becomes smooth, i.e. the track center becomes a perfect circle.
Conventional ZAP methods are very effective in reducing the RRO and AC track squeeze in a hard disc drive. However, as densities of disc drives increase, the amount of time required to perform conventional ZAP methods similarly increases. The ZAP time includes RRO data collection time, ZAP compensation table computation time, head seek time and ZAP table writing (onto the disc) time. The ZAP time increases rapidly when the disc density, or tracks-per-inch (TPI), goes higher and higher. To avoid limitations on the track pitch, a system is needed to compensate for repeatable runout errors, while at the same time reducing the time required for such compensation. In addition, techniques are needed to mitigate track squeeze issues that may be introduced by such compensation time reduction. The present invention provides a solution to these and other problems, and offers other advantages over previous solutions.